There is a popular belief in Japan that a person's ABO blood type or ketsueki-gata is predictive of their personality, temperament, and compatibility with others, similar to the Western world's astrology. I found it interesting so- I am AB.
It's not weird to not know your own blood type. I didn't know mine until I donated blood during my senior year of high school. Anyway, my blood type's O-positive, and I'm none of the things it says people with O-type blood are at all.
I knew my blood type when I was younger but as I grew older I forgot. I know I must be pretty dumb to forget it, but I didn't really see a use to know it then. I actually knew about this Japanese blood type thing though. haha. I came across it a while ago when I was studying some culture. It's very interesting.